Automatic darkening filters, or ADFs, are often used for applications like welding where protection from intense levels of incident light, such as the glare of a welding arc, is desired. A typical ADF includes electronic control circuitry, powered by a battery, which causes the filter to change from a light (clear or transparent) state when not subjected to the glare of the welding arc to a dark (nearly opaque) state upon exposure to such glare. This enables a welder to perform a welding operation and also perform tasks outside the welding area without removing the protective shield.
Conventional ADFs include layers of liquid crystal material capable of changing from a light state to a dark state under control of a control voltage. A sensor detects the start of a welding arc and generates a corresponding control voltage which, when applied to the filter lens, causes it to change from a light state to a dark state. Because the arc is already switched on when the sensors react, the switching of the ADF has to be very short, e.g., less than a few hundred microseconds. This abrupt or “hard” transition between the light state and the dark state can be uncomfortable to the user, especially under working conditions where many light-to-dark transitions are experienced throughout the course of a typical work day.
The sensors in a conventional ADF may be adversely affected by interference from other light sources, other welding machines, currents, or magnetic fields in the vicinity, which could cause the ADF to enter the dark state in the absence of a welding arc. In certain applications—such as low current tungsten inert gas (TIG) welding—the usable signal from the welding arc is relatively weak. In these cases, the detector may fail to detect the arc, resulting in failure of the ADF to enter the dark state even in the presence of a welding arc.